The invention relates to an arrangement for determining a transmission delay in a subscriber network.
The solution according to the invention is especially suitable for a measurement of timings of uplink messages (from a subscriber towards a central unit) of a point-to-multipoint network implemented by time-division technique. Such networks can be, for instance, combined cable television and telephone networks, so-called Passive Optical Networks (PONs) and wireless local loops.
In known subscriber networks based on the time-division technique, no subscriber-terminal-specific measurement of transmission delay is needed, for the connections are point-to-point connections. When the time-division technique is used in combined cable television and telephone networks or PONs, it is necessary to have a transmission delay for each subscriber terminal under control, because of the multipoint character of the connections. This means that a procedure of some kind has to be arranged in such a network for measuring the subscriber-specific transmission delay so that the subscriber terminals are able to learn the correct moments of transmission.
One known method of measuring a transmission delay is based on the looping of the signal to be transmitted outside an actual transmission channel. In these arrangements, the delay is measured by monitoring an own message, which is looped back from the other end of the connection. The transmission takes place outside the actual transmission channel, on a channel especially allocated for delay measurement. The looping can be made at the midpoint of the point-to-multipoint network, in which case a subscriber terminal measures the delay, or the subscriber terminal can be connected to form a loop, in which case a device at the midpoint of the network measures the loop delay.
Another known method of measuring a loop delay is a measurement to be performed on the actual transmission channel. Then, to a payload signal is added a low-level slow-changing message of DS-spread-spectrumsignal type (DS, standing for direct sequence), which message can be detected on the reception side by means of a specific correlation receiver. A low-level signal does not interfere a data transmission on the same channel, but allows a determination of the loop delay anyway.
Drawbacks of the known measuring methods are, e.g., separate device arrangements required, which make the equipment more complicated than before, and also the transmission capacity needed, which makes the bandwidth left for actual utility purposes smaller. A further drawback of the measurements to be performed outside the actual transmission channel is that the delay may vary in different frequency ranges.